1. Field of the Invention
The present invention relates generally to a method and apparatus for transmitting packets in a mobile telecommunication system, and in particular, to a method and apparatus for controlling transmission of data packets from an AN (Access Network) according to forward channel conditions by an AT (Access Terminal) in both a link adaptation scheme and an ARQ (Automatic Repeat reQuest) scheme.
2. Description of the Related Art
Mobile telecommunication systems typically include ATs and ANs. As a result of attenuation variations along a propagation path, severe inter-system interference, and fading according to a path distance and shadowing, the carrier-to-interference ratio (C/I) of a radio channel may significantly change depending on channel conditions. Link adaptation is a scheme of adapting a data rate to channel conditions such as received C/I in order to increase channel throughput. In the link adaptation scheme, the data rate is determined according to a code rate and a modulation. A link adaptation system increases a data rate by means of high code rate codes and high-level modulation when a received C/I is high. If the received C/I is low, the link adaptation system decreases the data rate by means of low code rate codes and low level modulation and retransmits packets selectively to thereby increase channel reliability.
In an HDR (High Data Rate) system standardized by the 3GPP2 (3rd Generation Partnership Project 2), physical layers using link adaptation on the forward link can be transmitted in 13 ways according to three modulation schemes, QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary Phase Shift Keying), and 16QAM (16-ary Quadrature Amplitude Modulation), three code rates, ¼, ⅜ and ½, and the number of slots in which a packet is repeatedly transmitted.
FIG. 1 illustrates HDR forward and reverse links. Referring to FIG. 1, an HDR packet includes 2048 chips per slot and a pilot channel in each half slot is assigned to 96 chips. Since the pilot channel is transmitted with the same power as that of a traffic channel, the C/I of the pilot channel is equal to that of the traffic channel. The pilot C/I becomes a criterion by which a code rate and modulation are determined.
FIG. 2 is a flowchart illustrating a packet transmission requesting procedure in a conventional HDR system. Referring to FIG. 2, upon receipt of a packet in each slot, an AT analyzes the preamble of the packet, checks whether the packet is destined for the AT, and if it is, checks whether an AN transmitted the packet at a data rate requested by the AT by detecting the length of the preamble in step S110. The AT measures the received C/I of a pilot signal in the packet in step S120 and determines a data rate corresponding to the received C/I in step S130. The AT feeds back information about the determined data rate in step S140. Here, the data rate information is called DRC (Data Rate Control) information, which is transmitted in each slot on a reverse DRC channel as shown in FIG. 1.
If the determined data rate is low, the AN transmits the packet repeatedly to ensure channel reliability. FIG. 3 illustrates packet lengths versus data rates on the HDR forward link. Referring to FIG. 3, the same packet is transmitted 16 times at 38.4 kbps and 8 times at 76.8 kbps. The packet is transmitted once at a high data rate of from 614.4 through 2457.6 kbps.
Despite the advantage of increased channel reliability, the repeated transmission of one packet results in the increase of channel estimation errors, slow adaptation of a code rate and modulation to channel changes, and dissipation of radio resources because one packet occupies a long slot period. In addition, since the HDR forward link is subject to TDM (Time Division Multiplexing) between users, if the users occupy many time slots at low data rates, the overall throughput is reduced.
On the other hand, in the case of ARQ, the AT performs a CRC (Cyclic Redundancy Code) check on a received packet and requests packet retransmission to the AN only if the packet has errors. Therefore, a data rate is virtually decreased and channel reliability is increased. In an advanced hybrid ARQ scheme, packet reliability is further increased by reducing a code rate in packet retransmission using error correction codes or by decoding packets of the same sequence number in combination.
FIG. 4 is a flowchart illustrating a packet retransmission requesting procedure in a conventional hybrid ARQ (HARQ) system. Referring to FIG. 4, an AT receives a packet in step S210 and accumulates packet symbols by combining the received packet with previously received packets of the same sequence number in step S220. The AT decodes the packet symbols and performs a CRC check in step S230 and determines whether CRC errors exist in step S240. If no CRC errors are found, the AT transmits an ACK (Acknowledgment) signal to an AN and the decoded data to an upper layer in step S250. The decoded data is processed in the upper layer and the accumulated symbols are discarded in step S260. On the other hand, if CRC errors are found, the AT requests retransmission of the same packet to the AN in step S245.
As described above, the CRC check is performed after decoding a received packet in ARQ. If a channel condition is very bad, errors will be detected continuously in the CRC check and retransmission will be requested continuously. Therefore, much power is dissipated for repeated decoding and a feedback delay is prolonged as much time as required for the decoding. This implies that the AT needs a memory of capacity large enough to store many packets.
Link adaptation and ARQ increase channel throughput by adapting a transmission scheme and the number of transmission times to channel conditions. Yet, the former may decrease the throughput by repeated packet transmission, whereas the latter has the problems of power dissipation, increased delay time, and the requirement of a large capacity memory.